You can share your unique coupon code across your social media pages for customers to use against your shop.
If you want organic visitors to your shop to know about your coupon simply go to Shop Manager > Settings > Store and tick enable vacation mode.
The Vacation Message will then appear and you can leave the coupon code in the message.
This will show the coupon code on all of your products and shop for customers to see.
Please note: Do not "Disable add to cart for my products" as this will put your shop in full vacation mode and customers will not be able to buy your products.